We are working on the fabrication of Hall devices from exfoliated flakes of 2D materials. We are exfoliating these 2D materials on polymer stamps (PDMS) and then deterministically transferring them on pre-patterned Hall geometry devices(Ti/Au contacts 5/70nm respectively) on SiO2/Si substrate using 2D dry transfer setup. After the successful transfer of 2D flakes on device geometry, we inspect the devices microscopically (in high-resolution 100X). Optically the 2D flake appears to be in perfect physical contact with all contact leads. But when we are characterizing them electrically, in the first 2-3 sets of I-V measurements, we are getting the expected resistivity range and also the Ohmic nature of our contacts. But after 2-3 sets of measurements, the resistance of the sample reaches to giga-ohm range, and also, the contacts are losing their ohmic nature.
While inspecting the device optically after IV measurements, it appears in perfect working order (i.e. the flake is still in the same condition as it was before the measurements).
It would be a great help if you could suggest something regarding the issue based on thhe expertise in bottom contact devices.
Hi,
It seems that your material is not stable. Either due to environmental affects, electrical characterization affects or both are causing some changes in the material. This may be reflected as the change in resistivity of the material.
Check whether your 2D material is responding to humidity, gases or organic vapors. Usually they do. This you can perform as soon as you transfer it on the SiO2/Si substrate.
Regards
Amit
Amit Morarka
Respected Sir, thank you for your enlightening reply. We are working with 2D materials (Graphene, Mos2, PtSe2), which are not sensitive to atmospheric conditions, as reported in the literature. Even though our devices are encapsulated with h-BN (from 2D semiconductors) and all fabrication (exfoliation and dry transfer of 2D flakes on device geometry) work is carried out inside the glove box.
From our basic analysis, we doubt the interface quality (maybe a large vdW gap leads to an extra vacuum barrier for carrier transport) between the 2D flake and gold film of the contacts; due to that, the capacitive behavior is coming in electric measurements. How can we improve the interface quality without affecting the pristine quality of our 2D flake?
Any suggestion regarding the issue is highly appreciated.
Hi,
It is this part of your query "But after 2-3 sets of measurements, the resistance of the sample reaches to giga-ohm range, and also, the contacts are losing their ohmic nature." which is in general an outcome of an instability of the material. Nevertheless I guess you have cover that ground meticulously.
As mentioned in my previous "Enlightening Reply", action of electrical characterization and as suggested by you, the transfer of 2D material was done using PDMS stamps assisted lifting and depositing process. Any residual remanence (Angstroms to nanometers in dimensions) of an uncured PDMS/organic solvents/unwanted material (Using glow box does not mean it is always clean!!! What is the clean room class of the glove box?) on the 2D material could change its properties over time under the action of repeated exposure to the applied electric field (electrical characterization). Hence, the electrical breakdown of the foreign material between the 2D flake and the Gold contact film. Such Deposited foreign material (if you are lucky) could be seen under the microscope that you are using. Most of the time, it is not visible under optical microscope.
Regarding your comment "We are working with 2D materials (Graphene, Mos2, PtSe2), which are not sensitive to atmospheric conditions, as reported in the literature."
Here are few examples of the above cited 2D materials (If they are the actual materials you are really working on!!) which shows good results in gas sensing applications.
1] Article High-Performance Hybrid Electronic Devices from Layered PtSe...
2] Article MoS 2 -Based Optoelectronic Gas Sensor with Sub-Part-per-Bil...
The approach of physically transferring 2D material is not standardized yet!!! It requires rigorous work to understand what does the substrate/contacts and then the deposited material is doing, all independently?
As for the doubt of appearance of the capacitive component in the latter stage of the measurement, if it is repetitively occurring with the exact same nature with numerous different devices, exactly after the laps of same amount of time and characterization cycle, you may have a new finding. If it is arbitrary in nature, there is something changing (unwanted foreign body) within the contacts and the 2D materials.
The closet solution you may try is thoroughly cleaning the surfaces before deposition. Make sure the PDMS stamp is completely cured. The transfer should take place just by the use of electrostatic interaction between the PDMS surface and the 2D material!!!
Regards
Amit
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